The present invention relates to an electric compressor used in a freezer, refrigerator, or air-conditioner. More particularly it relates to a highly efficient electric compressor in which loss torque and iron loss due to magnetic attraction are reduced. The magnetic attraction is produced at a bearing in a compressing section jointed to a motor section of the compressor.
A prior art is described with reference to FIG. 7 where a reciprocal electric compressor is shown.
In FIG. 7, the compressor comprises hermetic container 1, compressing section 2 disposed at lower part of the container, and motor section 3 disposed above the compressing section. Shaft 4 mounted to rotor 14 of motor section 3 has crank 4a on its tip.
Cylinder block 5 formed of a casting made of iron system material comprises bearing 6, in which shaft 4 is inserted, and cylinder 7 formed at right angles with bearing 6.
Piston 9 is linked to crank 4a via connecting rod 8. When motor section 3 is driven, rotating movement of shaft 4 is converted to reciprocal movement by crank 4a, and delivered to piston 9 via rod 8, so that piston 9 slides with respect to inner wall of cylinder 7. Compressing chamber 10 is formed by cylinder 7 and piston 9. Oil pipe 11 is mounted to a tip of crank 4a, and lubricant 12 pooled at the bottom of hermetic container 1 is lubricated to compressing section 2 and shaft 4 through oil pipe 11, so that respective sliding sections move smoothly.
Motor 3 is a two-pole induction motor comprising the following elements: (a) stator 13 formed of winding-wound-iron-core made of laminated magnetic sheets, and (b) rotor 14 formed of rotor iron core 15 with a secondary conductor, the rotor iron core being made of laminated magnetic sheets.
Bored section 16 is provided at the end of rotor-iron-core 15 on the side of compressing section 2, and bearing 6 extends inside bored section 16.
An operation of the conventional reciprocal compressor of which structure is discussed above is described hereinafter.
When rotor 14 spins, piston 9 performs reciprocal movement via connecting rod 8 linked to crank 4a of shaft 4, so that piston 9 compresses coolant gas in compressing chamber 10. The compressed gas is discharged through a discharging pipe (not shown) to a system such as a freezer, refrigerator, or air-conditioner.
Regarding the lubrication to respective sliding sections such as bearing 6, cylinder 7, connecting rod 8 and piston 9 of compressing section 2, oil pipe 11 mounted to lower end of shaft 4 rotates and pumps up lubricant 12 for lubrication.
Recently, reducing the power consumption of freezers, refrigerators, and air-conditioners has drawn attention because of energy saving tendency, and lower profiles of those apparatuses have been studied because of downsizing requirement. The rotor is disposed as close as possible to the compressing section, and a part of bearing extends inside the bored section, thereby regulating undesirable rotating deflection of the rotor and lowering the total height of the compressor. Thus the downsizing requirement is satisfied. However, power saving of the motor, which consumes the largest power in the freezing system, has not yet arrived at a satisfactory level.
In the conventional two-pole induction motor used in compressors, magnetic steel sheets of lower iron loss has been employed, a shape of the core has been optimized, or volume of materials used has been increased, in order to raise the efficiency of the motor. The induction motor, however, needs the exciting power for forming a magnetic circuit in addition to the power for producing torque as well as rotating load. Accordingly, efficiency improvement of the motor tends to be saturated, and it is difficult to expect a further substantial improvement of the efficiency.
A self-starting-synchronous-type two-pole motor using permanent magnets draws attention as another measures for increasing the efficiency of the motor. Because the permanent magnets are buried in the rotor, thereby eliminating the exciting power.
An example of this self-starting-synchronous-motor is described with reference to FIGS. 8 and 9. Regarding the entire compressor, only the motor is changed, and the changed points are detailed hereinafter.
Rotor 17 of the synchronous motor comprises iron core 18 made of laminated magnetic steel sheets and shaft hole 19 for receiving shaft 4 to fit into core 18. Bored section 20 is provided at the end of core 18 in an axial direction. It is not shown in the drawings, but a part of bearing 6 of a cylinder block 5 extends inside bored section 20.
Two pieces of plate-type permanent magnet 21 butt each other and form angle a to shape in a hill. Two pairs of these magnets 21 are inserted into rotor 17. A first pair of two magnets are placed such that S pole faces outside the rotor and N pole faces inside the rotor. A second pair of two magnets are placed such that N pole faces outside the rotor and S pole faces inside the rotor. Thus the first pair forms a rotor pole and the second pair forms another pole, so that entire rotor 17 has two poles. The width of magnet 21 is referred to as xe2x80x9cPxe2x80x9d.
A starter cage-shaped conductor is unitarily formed by aluminum diecasting comprising numbers of conductive bars 22 provided to core 18 and shorting grommets 23 covering both ends of core 18 in an axial direction.
Both end-faces of core 18 in the axial direction have protective terminal plates 24 made of non-magnetic material for securing magnets 21 from coming off. Barriers 25, for preventing magnetic flux between the permanent magnets from shorting, are provided to core 18. Barriers 25 are unitarily formed with the starter cage-shaped conductor by the aluminum diecasting.
The flow of magnetic flux from magnet 21 is schematically described with reference to FIG. 9 using the arrow marked lines. The magnetic flux from N poles of two magnets 21 placed at upper side of FIG. 9 travels mainly through the center section of core 18 and is attracted to S poles of two magnets 21 placed at lower side of FIG. 9. Thus the magnetic density through core section 18a around the outer rim 20a of bored section becomes substantially high.
As such, self-starting-synchronous type motor using permanent magnets can be used instead of the conventional induction motor. However, since bearing 6 made of iron-system material exists inside bored section 20, magnetic attraction works between the inner wall of bored section excited and the outer wall of bearing 6. The magnetic attraction produces loss torque which lowers the torque produced by the motor, and yet, magnetic flux of magnet 21 travels to bearing 6 and produces eddy-current-loss. The motor needs another power to compensate the loss torque and eddy-current-loss in order to continue operating, and this prevents the efficiency from increasing.
The present invention addresses the problem discussed above, and aims to provide a highly efficient electric compressor in which loss torque due to magnetic attraction and iron loss (particularly eddy-current-loss) in the bearing are reduced.
The compressor of the present invention comprises the following elements:
a compressing section accommodated in a hermetic container; and
a motor section for driving the compressing section and coupled to the compressing section.
The motor section includes a motor of two rotor poles, and the motor has a bored section at an end on the compressing section side and a rotor core in which permanent magnets are buried. The compressing section includes a bearing, made of non-magnetic material, extending inside the bored section.
This structure allows magnetic attraction not to work between an inner wall of the bored section and an outer wall of the bearing, so that no loss torque is produced. Since the bearing is made of non-magnetic material, magnetic flux from the permanent magnets are not attracted to the bearing and almost all the magnetic flux travels through the rotor core. Thus iron loss (particularly, eddy-current-loss) is rarely produced within the bearing. As a result, high efficiency of the motor is directly reflected to the compressor.
Another compressor comprises the following elements:
a motor section including two rotor-poles, a rotor core in which permanent magnets are buried, and a bored section at an end on a compressing section side;
a compressing section including a bearing, a part of which extends inside the bored section and at least the part is made of non-magnetic material.
This structure allows the magnetic attraction not to work between the inner wall of the bored section and the outer wall of the bearing, so that the loss torque is not produced. Further, the iron loss, the eddy-current-loss in particular, within the bearing due to the magnetic flux from the permanent magnets is prevented from being produced. In addition, inexpensive iron-system material can be used for the construction except for the extending portion of the bearing inside the bored section. The bearing can be unitarily formed with a cylinder block, thus a highly efficient and inexpensive compressor can be provided.
Still another compressor comprises the following elements:
a motor section including a two-pole rotor, rotor core in which permanent magnets are buried;
a compressing section including a bearing made of iron-system material,
and the rotor core faces the bearing via annular space in a radius direction in the bored section.
The annular space allows magnetic flux on the rotor side to rarely flow to the bearing. Thus even the bearing is made of iron-system material, neither loss torque nor iron loss, eddy-current-loss in particular, in the bearing is produced. Therefore, efficiency of the motor is directly reflected to the compressor. Further, since the bearing can be made of iron casting, and formed unitarily with other sections, an inexpensive compressor can be provided.